Various types of light sources, such as sunlight, fluorescent lamps, and incandescent lamps are used for recording a video of a subject using a color video camera. When the same subject is illuminated with different light sources, it appears almost same to human eye. However, the spectral distribution of light measured by a physical means is substantially different with each type of the light source. For example, white paper appears white under light from any type of light source. This is referred to as color adaptation of human eye.
On the other hand, color video cameras use physical light measurement, and this correction is necessary for matching the color adaptation of human eyes. A subject which appears white to human eyes must always be reproduced correctly white under any lighting condition. If a white subject is reproduced white to human eyes, colors will be reproduced correctly in visual sense. Adjusting the color video camera so that white can be correctly reproduced is referred to as "white balance adjustment".
Heretofore, white balance adjustment usually has been made manually. However, this adjustment is very troublesome for the user, and it is difficult to obtain the correct adjustment. This has been an obstacle to popularization of home-use color video.
Recently, an automatic white balance adjusting system which automatically adjusts white balance prior to recording video has been developed and marketed. This system utilizes the principle that white balance is correct if the differential color signal is zero when a white subject it taken. Thus, the different color signal is automatically adjusted to zero by taking a white subject.